This commit permits to fix several bugs in the imagemanager module.
This commit permits to fix several bugs in the imagemanager module.

The following problems are fixed:
* Add a specific behaviour for the identifier and image_list_identifier tags
* Manage correctly attributes -- it uses now JSON data for storing attributes
* The image properties are renamed to comply with cloudkeeper-one

Pansanel/cloudkeeper-os,Mamut3D-docker/cloudkeeper-os